[X86] combineTruncate - trunc(srl(load(p),amt)) -> load(p+amt/8) - ensure there isn't an interdependency between the load and amt (llvm#165850)

Fixes llvm#165755

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -54768,9 +54768,11 @@ static SDValue combineTruncate(SDNode *N, SelectionDAG &DAG,
       KnownBits KnownAmt = DAG.computeKnownBits(ShAmt);
       // Check the shift amount is byte aligned.
       // Check the truncation doesn't use any shifted in (zero) top bits.
+      // Check the shift amount doesn't depend on the original load.
       if (KnownAmt.countMinTrailingZeros() >= 3 &&
           KnownAmt.getMaxValue().ule(SrcVT.getSizeInBits() -
-                                     VT.getSizeInBits())) {
+                                     VT.getSizeInBits()) &&
+          !Ld->isPredecessorOf(ShAmt.getNode())) {
         EVT PtrVT = Ld->getBasePtr().getValueType();
         SDValue PtrBitOfs = DAG.getZExtOrTrunc(ShAmt, DL, PtrVT);
         SDValue PtrByteOfs =

llvm/test/CodeGen/X86/pr165755.ll

@@ -0,0 +1,26 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 6
+; RUN: llc < %s -mtriple=i686-- | FileCheck %s --check-prefixes=X86
+; RUN: llc < %s -mtriple=x86_64-- | FileCheck %s --check-prefixes=X64
+
+define i32 @PR165755(ptr %p0) {
+; X86-LABEL: PR165755:
+; X86:       # %bb.0:
+; X86-NEXT:    movl {{[0-9]+}}(%esp), %ecx
+; X86-NEXT:    movl (%ecx), %eax
+; X86-NEXT:    movb $0, (%ecx)
+; X86-NEXT:    retl
+;
+; X64-LABEL: PR165755:
+; X64:       # %bb.0:
+; X64-NEXT:    movl (%rdi), %eax
+; X64-NEXT:    movb $0, (%rdi)
+; X64-NEXT:    retq
+  %ld64 = load i64, ptr %p0, align 8
+  store i8 0, ptr %p0, align 1
+  %ld32 = load i32, ptr %p0, align 8
+  %mask = and i32 %ld32, 32
+  %zext = zext i32 %mask to i64
+  %srl = lshr i64 %ld64, %zext
+  %res = trunc i64 %srl to i32
+  ret i32 %res
+}